AISC 13th Ed. and CJP DW Moment Connections 2

[LowLax]

We have a job in the office where we are working for the steel fabricator to design the moment connections for a project. The EOR has asked that all moment connections be designed for full moment strength of each beam.

The moment connections are directly welded (DW) moment connections. Beam flanges are field welded to column flanges with CJP welds. Rat holes and backer bars are used. Shear is taken care of by a single plate connection.

My question is: Is it possible for this type of connection to be strong enough using the 2005 AISC specification?

It used to be we didn't check anything with regard to the weld if it was CJP. Now the specification includes equations; J2-2 and J2-3. It specifically ask for the base metal to be checked. Although when you go to table 2.5 for the omega factor (we are using ASD), there are no values given under CJP welds, it just says strength is controlled by base metal.

Since there is a rat hole next to the beam flange. I suppose I have to check the flange as a plate for tension yielding.

Here is an example of what I'm running into:

Beam: W18x46
Maximum Moment: 226 k-ft
d = 18.1 in
tf = 0.605 in
bf = 6.06 in
Flange force = 226*12/(18.1-0.605) = 155.02 kip
Tension Yielding of Flange = 0.605*6.06*50/1.67 = 109.77 kip
109.77<155.02 N.G.

Is this going to be the case for every beam? Is it impossible to develop the full moment capacity of a beam with this type of connection?

 

[nutte]

Practically speaking, this connection will develop the beam's moment capacity, and there are no checks to be done.

Technically speaking, as your post points out, it would appear this connection doesn't really develop the full moment capacity, since it ignores the flexural capacity of the web.

Yet, this is still the way it's always been done. The 3rd edition LRFD manual says on page 12-6 that "the plastic moment of the supported beam can be developed with sufficient inelastic rotation and deformation capacity through such a connection. This apparent increase in beam strength above the prediction of elastic theory occurs because of strain hardening in the flanges."

 

[LowLax]

I posed the same question to AISC and here is their response (seems to echo a lot of nutte's points):

I will answer the question posed "Is it possible for this type of connection to be strong enough using the 2005 AISC specification?" in general terms and neglect the particulars of your case.



The short answer is "Yes, it is possible to develop the design moment capacity of a beam by direct CJP of the flanges alone."



AISC's Design Guide 13: Wide-Flange Column Stiffening at Moment Connections comments on the practice of developing the full moment capacity of the beam through a flange only connection as follows:


"However, when the full flexural strength of the beam must be developed, or when the axial force is large, such a model seems to guarantee an overstress in the beam flange, particularly for a directly welded flange moment connection. Nonetheless, the above force transfer model remains acceptable because inelastic action into the range of strain hardening allows the development of the design flexural strength of the beam in the connection (Huang et al., 1973). Such self-limiting inelastic action is permitted in LRFD Specification Section B9. Alternatively, a web connection with a stiffness that is compatible with that of the connections of the beam flanges can be used to activate the full beam cross-section and reduce the portion carried by the flanges."

Huang et al showed that the full calculated moment capacity of the section could be developed with only the flanges attached to deliver the moment. The calculated moment capacity of course neglects the effects of strain hardening, so the final design is consistent with the assumptions made.

--end quote

I think they are saying it works and you don't have to worry about checking the flanges for yielding. This is because the flanges can yield and develop strain hardening and there is extra stiffness in the shear connection. I wish they would just put this in the code somewhere.

 

[Lion06]

I would also note that in your above calculations, you should be dividing by 1.5 for "Tension Yielding of Flange", not 1.67. This is ultimately a bending problem and the Mr you are using reflects a factor of safety of 1.5, therefore, you should be consistent when doing the rest of the comparison.
I also noted that the Mr for a W18x46 if 217 K-ft, not 226. This is from my green book, I don't have my 13th edition manual home with me so there might be that slight difference.

 

[LowLax]

StructuralEIT,

I wish I could use those numbers I might get a better result. I'm using the Mp assuming the beam is fully braced, which in the 13th edition is 226 k-ft. Actually the omega factor for bending and tension yielding are both 1.67.

This all came up because there is a certain software we are using in the office that has recently been upgraded to a new version. The old version used to say something like "CJP no need to check". But now the new version runs a check on the flange like it is a pure plate in tension receiving a tension force equal to the maximum moment divided by d-tf. This is all logical, but I think it's overly conservative and I think AISC agrees. However I have not been able to convince the software company.

I'm sure many connections like this have been done through the years and I bet many engineers assume full moment strength is being developed. I suppose someone could argue you have to add extra plates on the flanges and full pen weld those too. The fabricators would love that one.

 

[Lion06]

Our typical detail shows a full pen weld at both flanges and a typical web shear connection.
Like you state, based on the software upgrade you will ALWAYS get a warning for a full moment connection (if it needs to develop the full moment of the beam.
As you found out from AISC, don't worry about it.